1. Field of the Invention
The invention relates to a proportional-integral-derivative (PID) control system.
2. Description of Related Art
There has been known a hydraulic power steering system in which hydraulic fluid from a hydraulic pump is supplied through a hydraulic control valve to a power cylinder connected to a steering mechanism of a vehicle, whereby steering is assisted. In a commonly-used hydraulic power steering system, a hydraulic control valve is mechanically connected to a steering member, such as a steering wheel, via a steering shaft, and an opening degree of the hydraulic control valve is adjusted in response to an operation of the steering member. Refer to, for example, Japanese Patent Application Publication No. 2006-325313 (JP 2006-325313 A), Japanese Patent Application Publication No. 2004-130899 (JP 2004-130899 A), Japanese Patent Application Publication No. 6-138905 (JP 6-138905 A), and Japanese Patent Application Publication No. 2008-506587 (JP 2008-506587 A).
There has been developed a hydraulic power steering system in which a hydraulic control valve is not mechanically connected to a steering member, and an opening degree of the hydraulic control valve is controlled by an electric motor (valve driving motor). The inventors et al. have conducted research on a system that controls a valve driving motor through PID control, and finally made the invention.